The present invention relates to magnetic resonance imaging for obtaining MR (magnetic resonance) images of an object based on the magnetic resonance phenomenon of spins of the object, and in particular to a switchover technique of signals or coils when MR signals received by an RF coil are given a signal processing apparatus for reception.
A particularly preferred example of the present invention is a switchover technique of signals or coils when MR signals received by a plurality of element coils composing a multi-coil serving as a receiving RF coil are given a signal processing apparatus for reception. Such a multi-coil is called a phased array coil (PAC). Another preferred example is to apply the above switchover technique to fast MR imaging that uses the foregoing multi-coil to conduct imaging at higher speed (hereafter, such imaging is called parallel MR imaging).
(Related Art)
Magnetic resonance imaging can be summarized as an imaging technique whereby nuclear spins of an object placed in a static magnetic field are magnetically excited by applying an RF signal of the Larmor frequency to the object and MR signals induced responsively to the excitation are used for reconstructing images.
In the field of this magnetic resonance imaging, a particularly interested imaging technique is fast imaging hat has been studied actively in recent years. One such example of the fast imaging is carried out using a multi-coil that consists of a plurality of RF coils (referred to as element coils). Such fast imaging is generally known as “parallel MR imaging.” In the historical view point, the parallel MR imaging was also called multiple-coil fast imaging, a PPA (Partially Parallel Acquisition) technique or a subencoding technique.
The parallel MR imaging can be reduced in practice in various types of schemes. In the early stage, there were the imaging schemes proposed by (1): “Carlson J. W. and Minemura T., Image Time Reduction Through Multiple Receiver Coil Data Acquisition and Image Reconstruction, MRM 29:681–688, 1993” and (2): “Ra J. B. and Rim C. Y., Fast Imaging Using Subencoding Data Sets From Multiple Detectors, MRM 30:142–145, 1993.”
In addition, there were proposed many other imaging schemes improved from the early ones. Such imaging schemes include a SMASH technique proposed by (3): “Sodikson D. K. and Manning W. J., Simultaneous Acquisition of Spatial Harmonics (SMASH): Fast Imaging with Radiofrequency Coil Arrays, MRM 38:591–603, 1997” or others; a SENSE technique known by (4): “Pruessman K. P., Weiger M., Scheidegger M. B., and Boesiger P., SENSE: Sensitivity Encoding for Fast MRI, MRM 42:952–962, 1999”; and a technique based on (5): “M. A. Griswold, P. M. Jakob, M. Nittka, J. W. Goldfarb and A. Haase, Partially Parallel Imaging with Localized Sensitivities (PILS), ISMRM 2000, p. 273.”
Though there is a little difference scheme by scheme, the basic concept of the parallel imaging is the same. That is, a multi-coil that consists of a plurality of RF coils (element coils) is used to simultaneously receive MR signals from those RF coils, and independent image data is produced from an echo signal received by each element coil. On condition that the simultaneous reception is performed through the plural RF coils, the number of times of encoding to each RF coil is reduced to an amount calculated by dividing a predetermined number of times of encoding for image reconstruction by the number of RF coils. Hence the FOV (field of view) of an image from each RF coil becomes small, but the scan time reduces.
There are, however, caused folding phenomena (or called wrap-around) at edges of each image. To remove this, by making use of the fact that the plurality of RF coils differ in sensitivity from each other, the parallel MR imaging adopts unfolding processing taken place as post-processing on a plurality of images each obtained from each RF coil. Practically, the unfolding processing is carried out with the use of the spatial sensitivity map of each RF coil.
Since the spatial sensitivity map is changed if the size of an object and/or electrical loads differ, it is more frequent that calibration data is acquired and updated every time a new patient is examined. Continuing to use calibration data once acquired over a plurality of patients is not so welcomed because of such reasons. The acquisition techniques of calibration data include “an independent scan technique” whereby a scan for sensitivity maps is performed between examinations independently from main scans and “a self-calibration technique” whereby a scan for obtaining sensitivity maps is additionally incorporated in each main scan.
The plural images that have been subjected to the unfolding processing are then combined into a final image of which FOV covers a desired full area. Hence, this parallel MR imaging makes it possible that the scanning is made faster (i.e., fast imaging) and a wide-view image such as an image covering the entire abdominal area is provided.
Incidentally, of the imaging techniques listed by the foregoing references (1) to (5), the imaging techniques provided by the references (1) and (3) are dedicated to particularly shaped element coils, while the technique provided by the reference (2) is dependent of the shapes of element coils, thus being generalized. The imaging technique proposed by the reference (4) has been developed from that proposed by the reference (2).
The imaging technique provided by the reference (5) is based on combining sum-of-square images, which is stable under only particular conditions.
A recent trend in the field of such parallel MR imaging is to raise the number Ncoil of element coils (RF coils) to be used in order to meet a growing demand for faster imaging. In the conventional, the more the number of element coils, the more the number Nch of channels installed in a reception apparatus needed for processing the received MR signals. As long as the number Nch is equal or larger to or than the number Ncoil, the signals detected by the element coils can be taken into the reception apparatus, respectively.
However, since it is generally true that increasing the number Nch of channels will lead to a rise in the manufacturing cost of the MRI system, the number Nch of channels is limited in the practical MRI systems. Although proper arrangement of element coils are normally dependent on imaging conditions (particularly, the encoding direction and the size of an FOV). Without such proper arrangement, the image quality may be deteriorated due to a decreased SNR or artifacts, or resulted in restricting the imaging conditions. Further, when the number of channels is increased, the volume of data that should be processed in the reception apparatus becomes larger as well, thereby the time required for the data processing being forced to be longer.
In cases where the abdomen of an object is subjected to parallel MR imaging, abdomen-dedicated RF coils are placed along each of the body surfaces of the abdomen and back. In other words, a pair of abdomen-dedicated RF coils is arranged with the objet's abdomen contained therebetween. By way of example, the abdomen-dedicated RF coil is a multi-coil, which is preferably formed into a QD (quadrature detection) coil made of a figure-of-eight-shaped coil combined with a loop coil.
Thus, for performing the parallel MR imaging with the use of the single QD coil, it is best preferred that a phase encoding direction for the imaging is set to a direction from the object's back to the object's abdomen. The reason is that, when the parallel MR imaging is performed sufficiently, it necessitates a plurality of element coils displaced in the phase encoding direction. As exemplified, the limitation of the phase encoding direction to a particular direction will lead to heavily limited imaging conditions, thus causing the problem that the imaging cannot be performed with priority given to the states of organs and blood flows in an object.